1. Field of the Invention
This invention relates to a method and apparatus for manufacturing a glass lens of high accuracy. It should be understood that the term glass lens also includes other optical elements made of glass such as a prism, etc.
2. Description of the Related Art
According to a prior art method of manufacturing a glass lens, the temperature of at least one of a paired upper and lower molds is detected. This mold is heated to a temperature higher than the transition point of a lens blank but lower than the softening point of a lens blank so as to heat the lens blank to a moldable temperature. Thereafter, a mold clamping force is applied for press molding the glass lens. In other words, the timing of applying the mold clamping force is determined by taking the mold temperature as a reference.
However, the thermal expansion coefficient of glass in a temperature range higher than the transition point is several times larger than in a temperature range lower than the transition point. Since the thermal expansion coefficient of glass in the high temperature range is substantially larger than that of the mold material, such as ceramics, metal, etc., when the glass lens is press molded in the high temperature range, although the press molded glass lens has a given accuracy, in the period between the press molding and a point at which the temperature of the molded lens is lower than the transition point, the molded lens shrinks greatly. The molds resulting in a so-called sink mark. This tendency increases as the diameter of the lens increases and greatly decreases the accuracy of the lens. Thus, elimination of the problem of forming sink marks has been an important factor for improving the accuracy of the lens. Although this problem can be solved by molding the lens without excessively increasing the temperature of the lens blank, as it is impossible to directly measure the blank temperature it is necessary to estimate the blank temperature from the mold temperature. Moreover, the temperature of the mold is different dependent upon the points at which the temperature is measured. Therefore, with a method of press molding the lens by taking the temperature of the mold as a reference, it is impossible to accurately control the temperature of the lens blank. Also, it has been impossible to satisfactorily press the molds while maintaining the molds at a lower temperature.